Means for preventing pulsations in centrifugal compressors



March 10, 1931. F. SCHMIDT 1,796,053

MEANS FOR PREVENTING PULSATIONS IN CENTRIFUGAL COMPRESSQBS Filed July 23. 1929 Z0 0 Z I, Z2 Z6 Z8 23 I ,8 0 J9 I9 I9 [5 2'5 2 122 352/ 22 2a Z5 Z2 2/ l 1 a Inventory Friedrich Schmidt, b M w ak His Attorney.

Patented Mar. 19, 1931 stares PATENT OFFICE FRIEDRICH SCHMIDT, OF BERLlN-KARLSHORST, GERMANY, ASSIGNOR TO GENERAL ELECTRIC COMPANY, A CORPORATION OF NEXV YORK MEANS FOR PREVENTING PULSATIONS IN CENTRIFUGAL COMPRESSOR-S Application filed July 23, 1929, Serial No.

This invention relates to centrifugal air compressors of the type in which a rapidly rotating impeller forces the air through curved or tangential passages in a stationary ring, usually termed a diffuser, surrounding saidimpeller.

The action of these passages is to convert the velocity of the air into pressure, and when properly designed such machine may, as in the present case, operate on what is known as a rising characteristic. This means that the pressure does not remain constant for all volumes of air flowing through the intake, but varies with the volume delivered. 7

The object of the present invention is to provide an improved method of and means for preventing the pulsations of the column of air or other gases flowing through the machine. In the following specification the compressor is described as a machine for handling air, but it will be understood that my invention is not limited to this use.

Pulsations are :found to occur under certain conditions of light load and are known to 5 depend on the volume of air which the machine is handling and on the pressure at which it is being delivered. In other words, for every volume there is a critical pressure above which pulsations are liable to occur,

and vice versa, for every pressure there is a critical volume below which pulsations are likely to occur. The critical pressures and volumes in different machines vary considerably, iowever, and for any particular machine they can only be determined by experiment. Pulsations or violent pressure fluctuations cause excessive noise and set up dangerous vibrations and shocks in the machine and prevent the delivery of a steady blast of air, which is the desired purpose of an air compressor.

It is well known that pulsations in a compressor may be prevented. at least to a great extent, by throttling the flow of air through it as, by throttling the intake, or by wasting the air either from one of the stages or the delivery end o l" the machine, or by by-passing it from the delivery side to the intake, or by the use of adjustable blades.

Wasting air, usually termed blowing off,

380,403, and in Germany August 3, 1928.

has the disadvantages of rendering the ma chine inefiicient and causing excessive noise. Adjustable blades are not effective in preventing pulsations when the machine is delivering less than 25% of its maximum output. Below this limit it is necessary to resort to blowing off. Adjustable blades further present considerable difficulty in mounting said blades on the rotor and cut down the efficiency of the machine.

The phenomenon'of pulsation is due to a large extent to the reduced velocity in the diffusers during reduced delivery. According to my invention I prevent the occurrence of pulsations by increasing the velocity of the air through the diffusers by introducing air at a higher pressure through nozzles, the air being caused to flow in the same general direction as the air discharged from the impellers. The air thus introduced may come from a higher pressure stage, the compressor discharge or an outside source.

In the accompanying drawing the figure is a central sectional view partially in elevation of a horizontal compressor equipped with my pulsation preventing means.

Referring to the drawing, 10 indicates a casing of a multistage centrifugal compressor having an intake 11 and a discharge 12. Located in said casing are spaced impellers 13 secured to a shaft 15 which is mounted in bearings 16. A throttling valve 18 is provided in the intake 11. The impellers l3 discharge air into diffusers 19 which are formed. in casing 10. The diffusers of the 2nd, 4th, 6th stages, and so on are modified as shown at 23 in the figure, in order to direct the flow of air through the intercoolers 24 (but one being shown in the figure). The air is returned from the intercoolers through passages 25 which form the intakes of the 3rd, 5th stages, and so on.

The air entering the intake 11 passes through an annular chamber 17 through the impeller of the 1st stage, thence through the 1st stage diffuser 19 to the 2nd stage impeller, then through the diffuser 23 and the intercooler 24 to the 3rd stage, the course of the air through the compressor and intercooler o ing elastic fluid to the diffuser of a selected being designated by arrows in the passages 23 and 25.

In order to increase the velocity of the air through the difi'users 19 I provide in the casing 10, adjacent and preceding the diffusers 19,. chambers 20. These chambers are closed except as hereinafter specified. I Nozzles 28 are cut through the wall separating the chambers 20,from the ditlusers 19, the nozzles ly-v ing in such a directionlthatair from chamber 20 is directed into the diffusers 19 inthe same general directionas the air thrown-from the impeller. By-pass pipes 21 each provided with a throttling'valve 22 are provided connecting the diffusers 23 with the chamber 20 adjacent the impeller of the preceding stage, excepting the last stage in which the pipe 21 i I leads' from the discharge conduit instead-of'a thatby-passing air through pipes 21 does not I 7 sure stage.

diffuser 23. In each case air from the discharge 12 or a stage of higher pressure flows through the nozzles 28 and the diflusers 19 in the same general direction as'the impeller air and-air thus introduced at a higher pressure increases the velocity in diffusers 19. It is obvious that the air may be introduced from an outside source instead of thehigher pres sure stages or the discharge.

A by-pass pipe 26 connects the diffuser 19 or the firststage with the annular chamber 17 which chamber forms a part of the intake.

- This pipe is provided with a throttling valve 27. If the deliveryof air is reduced so low prevent pulsation, blowing off may "be resorted to by opening valve 27 inp'ipe -26l Blowing off from the flow'pressure stage to the chamber 17 will not be noisy since the pressure at the point'is low. When the compressor is delivering a sulficient quantity of air so that pulsation will not occur valves 22 are closed. As the quantity of air delivered is reduced-any selected valve or v'alves22 may be opened. partially or1fully to prevent pulsations. The valves 1.22 may be manually or automaticallyoperated.

VVhatI claim as new and desire to secure by Letters Patent of v the United States is t:

'1- The method of preventing pulsations at light loads of'a multistage centrifugal elastic fluid compressor which consists in introducstage at a pressure which is higher than'normally exists in said stage in-orderto raise the velocitylof the fluid in asaidstage.

,stages,.an impeller and a diffuser for each stage, chambers adjacent the diflusers, nozzles opening fromnthechambers intothe dif-V fusers, conduits which receive fluid from stages of higher pressure and 'by-pass it to chambers respectively associated with stages of lower pressure,'and control valves for said conduits. V I a I Y 5. A multi-stage compressor comprising a plurality of centrifugally acting impellers which discharge gas radially outward -a 'dif-V fuser for each impeller receivinggastherefrom'and discharging it outwardly, gas discharging means located in a Wall of meet the difiusers and opening intoit'between'the inlet and exitthereof, andconduitzmeans for delivering gas to said discharging means from a selected stage of higher pressure, wherebythe flow of gas through the difi'user is accelerated.

In witness whereof, I have hereunto set my hand-this 11th day of July, 1929.

FRIEDRICH SCHMIDT.

2. A multistagecentrifugalaircompress I Y sorhaving a by-pass-pipe provided witha control valve therein and extending from the difiuser of a stage of higher pressure-to a plurality of individual nozzles formed in the diffuser casing ofa stage of lower pressure, the nozzles being adapted to deliver air in the same general direction as the air delivered from the impeller of said lower pres- 3. A multi stage centrifugal compressor 

